Method for controlling color-image forming apparatus

ABSTRACT

A method of controlling a color image forming apparatus, the method including: reducing the C, M, and Y channels from the boundary area of the composite black text, thereby allowing only the K channel to be left on the outermost boundary of the composite black text; and compensating for the K channel by the density of the reduced C, M, and Y areas, so that image distortion generated at the boundary of the composite black text is compensated for, resulting in increased image quality of the printed image of the composite black text.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims all benefits accruing under 35 U.S.C. §119 fromKorean Patent Application No. 2007-13211, filed on Feb. 8, 2007 in theKorean Intellectual Property Office, the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Aspects of the present invention relate to a method of controlling acolor image forming apparatus, and more particularly to a method ofcontrolling a color image forming apparatus to compensate for an outputimage distortion caused by mis-registration during the printingoperation of a color document.

2. Description of the Related Art

Generally, an image forming apparatus converts an image file (such as auser's document created by an application program or an image capturedby a user's digital camera) into encoded data, and prints the encodeddata on a print medium (such as paper) so that the user can view theimage. Examples of an image forming apparatus include a printer, aphotocopier, a facsimile machine, a multi-functional product, etc.

An image forming apparatus capable of printing a color image includes avariety of toners (e.g., Cyan, Magenta, Yellow, and Black toners).Different colors of the print data are implemented by the combination ofthe above-mentioned toners having different colors, and are then printedon the print medium.

Unlike a black-and-white printer (also called a mono printer), theabove-mentioned color image forming apparatus overwrites some colors ona single print medium several times in order to print a desired colorimage on the print medium. In this case, the color image formingapparatus has difficulty in correctly printing individual colors atdesired positions due to a variety of factors resulting from printing avariety of colors on a single print medium. This problem is hereinafterreferred to as a mis-registration.

Particularly, color distortion by which color dots disperse in alldirections becomes serious at a boundary between composite black textsdue to the mis-registration.

Generally, attributes of the print data are classified into an image, agraphic, and a text. Specifically, in the case of a composite blacktext, if the print data is determined to be a text, the image is printedin pure black. However, if the attributes of the print data aredetermined to be of an image, the black text is printed in compositecolors. For example, if the color image forming apparatus based on fourcolors (i.e., C, M, Y, and K) determines the attributes of the printdata to be of an image, the black text is printed in four colors (C, M,Y, and K). In this case, color dots having different colors may dispersein the vicinity of a text boundary due to the mis-registration, and thelocations of the C, M, Y, and K dots on the screen are different fromthose of the printed dots. In other words, the C, M, Y, and K dots maybe printed in the wrong locations due to mechanical errors resulting inthe dispersion of the color dots. Accordingly, the desired image isdistorted by the mis-registration, resulting in a deterioration of theprinted image quality.

SUMMARY OF THE INVENTION

Several aspects and example embodiments of the present invention providea method of controlling a color image forming apparatus to reduce animage distortion at the boundary of a composite black text resultingfrom a mis-registration, thereby increasing the quality of a printedimage.

Additional aspects and/or advantages of the invention will be set forthin part in the description which follows and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

According to an aspect of the present invention, there is provided amethod of controlling a color image forming apparatus that prints acomposite black text using cyan (C), magenta (M), yellow (Y), and black(K) channels, the method including: determining whether print datacorresponds to a composite black text; if the print data corresponds tothe composite black text, searching for a boundary area in the printdata; and reducing the C, M, and Y channels at the boundary area, andcompensating for the K channel by a density of the reduced C, M, and Ychannels.

According to another aspect of the present invention, there is provideda method of controlling a color image forming apparatus that prints acomposite black text using cyan (C), magenta (M), yellow (Y), and black(K) channels, the method including: determining whether patterns of theC, M, Y, and K channels in print data are identical to each other, anddetermining whether the K channel is flat; determining that the printdata corresponds to a composite black text if the patterns of the C, M,Y, and K channels are identical and the K channel is flat; if the printdata corresponds to the composite black text, searching for a boundaryarea in the print data; and reducing the C, M, and Y channels at theboundary area, and compensating for the K channel by a density of thereduced C, M, and Y channels.

According to another aspect of the present invention, there is provideda method of controlling a color image forming apparatus that prints acomposite black text using cyan (C), magenta (M), yellow (Y), and black(K) channels, the method including: searching for a boundary area inprint data if the print data is determined to correspond to a compositeblack text; and reducing the C channel, the M channel, and the Y channelat the boundary area, and compensating for the K channel by a density ofthe reduced C, M, and Y channels if the print data is determined tocorrespond to the composite black text.

According to another aspect of the present invention, there is provideda color image forming apparatus that prints a composite black text usingcyan (C), magenta (M), yellow (Y), and black (K) channels, the apparatusincluding: a control unit to search for a boundary area in print data ifthe print data is determined to correspond to a composite black text, toreduce the C channel, the M channel, and the Y channel at the boundaryarea, and to compensate for the K channel by a density of the reduced C,M, and Y channels.

In addition to the example embodiments and aspects as described above,further aspects and embodiments will be apparent by reference to thedrawings and by study of the following descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention will become apparentfrom the following detailed description of example embodiments and theclaims when read in connection with the accompanying drawings, allforming a part of the disclosure of this invention. While the followingwritten and illustrated disclosure focuses on disclosing exampleembodiments of the invention, it should be clearly understood that thesame is by way of illustration and example only and that the inventionis not limited thereto. The spirit and scope of the present inventionare limited only by the terms of the appended claims. The followingrepresents brief descriptions of the drawings, wherein:

FIG. 1 is a structural diagram illustrating an image forming apparatusaccording to an example embodiment of the present invention;

FIG. 2 is a flow chart illustrating a method of controlling a colorimage forming apparatus according to an example embodiment of thepresent invention;

FIG. 3 is a flow chart illustrating a method of determining a compositeblack text according to an example embodiment of the present invention;

FIG. 4 is a flow chart illustrating a method of determining a compositeblack text according to an example embodiment of the present invention;

FIG. 5 is a conceptual diagram illustrating a method of determining acomposite black text in a 3×3 window according to an example embodimentof the present invention;

FIG. 6 is a conceptual diagram illustrating a method of reducing C, M,and K areas at the boundary of the composite black text so that only theK channel is left at the outermost boundary according to an exampleembodiment of the present invention; and

FIG. 7 shows a comparison result between first data created before animage distortion compensation of the composite black text “quick” andsecond data created after the image distortion compensation of thecomposite black text “quick” according to an example embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The embodiments are described below in order to explain thepresent invention by referring to the figures.

FIG. 1 is a structural diagram illustrating an image forming apparatusaccording to an example embodiment of the present invention. Referringto FIG. 1, the color image forming apparatus includes a photoconductivedrum 1, a charged roller 2, an exposure unit 3, a developing cartridge4, an intermediate transfer belt 6, a first transfer roller 7, a secondtransfer roller 8, and a fixing unit 9.

The photoconductive drum 1 is configured such that a photoconductivelayer is formed at a circumference of a cylindrical metal drum thereof.It is understood that a photoconductive belt (not shown) may be usedinstead of the photoconductive drum 1. The charged roller 2 is adaptedto charge the photoconductive drum 1 with a uniform electric potential.The charged roller 2 may rotate simultaneously while contacting thecircumference of the photoconductive drum 1, and provides thephotoconductive drum 1 with electric charges so that the circumferenceof the photoconductive drum 1 is charged with a uniform electricity. Itis understood that a corona discharger (not shown) may be used insteadof the charged roller 2. The exposure unit 3 transmits an optical signalcorresponding to image information on the charged photoconductive drum 1so that an electrostatic latent image is formed thereon. The exposureunit may be a laser scanning unit (LSU) that uses a laser diode as alight source.

According to an aspect of the present invention, the color image formingapparatus uses four toners (i.e., cyan (C) toner, magenta (M) toner,yellow (Y) toner, and black (K) toner) to print the color image. For theconvenience of description and better understanding of the presentinvention, the above-mentioned four toners are denoted by referencecharacters “C,” “M,” “Y,” and “K,” respectively, to distinguishindividual toners from each other according to individual colors.

The color image forming apparatus includes four toner cartridges (11C,11M, 11Y, and 11K) including the C, M, Y, and K toners, respectively.Furthermore, the color image forming apparatus includes four developers(4Y, 4M, 4C, and 4K) receiving individual toners from the tonercartridges (11Y, 11M, 11C, and 11K), respectively, and developing anelectrostatic latent image formed on the photoconductive drum 1. Eachdeveloper 4 includes a developing roller 5 in a processing direction ofthe photoconductive drum 1. The developer 4 is spaced apart from thephotoconductive drum 1 by a predetermined distance corresponding to adeveloping gap. The developing gap may, although not necessarily, be setto several tens or hundreds of microns. In the case of a multipath-typecolor image forming apparatus, a plurality of developers 4 may besequentially operated.

A developing bias signal is applied to the developing roller of theselected developer (e.g., 4Y), and is not applied to the remainingdevelopers (e.g., 4M, 4C, and 4Y). Rather, a developing-prevention biassignal may be applied to the remaining developers (e.g., 4M, 4C, and4Y). Furthermore, only the developing roller 5 of the selected developer(e.g., 4Y) rotates, and the remaining developers (e.g., 4M, 4C, and 4K)do not rotate.

The intermediate transfer belt 6 is supported by supporting rollers 61and 62, and may travel with the same traveling linear velocity as therotational linear velocity of the photoconductive drum 1.

The length of the intermediary transfer belt 6 may be greater than orequal to the length of a maximum-sized print medium P used for the imageforming apparatus. The first transfer roller 7 is arranged to face thephotoconductive drum 1. A first transfer bias signal for copying thetoner image developed on the photoconductive drum 1 to the intermediatetransfer belt 6 is applied to the first transfer roller 7. The secondtransfer roller 8 is arranged to face the intermediate transfer belt 6and is spaced apart from the intermediate transfer belt 6 while thetoner image is transferred from the photoconductive drum 1 to theintermediate transfer belt 6. When the toner image is completelytransferred to the intermediate transfer belt 6, the second transferroller 8 contacts the intermediate transfer belt 6 at a predeterminedpressure. A second transfer bias for transferring the toner image to theprint medium P is applied to the second transfer roller 8. Then, acleaner 10 removes toner left on the photoconductive drum 1 after thetransferring action.

A method of forming a color image according to the above-mentionedconfiguration will hereinafter be described in detail. An optical signalcorresponding to Y-color image information is transferred from theexposure unit 3 to the photoconductive drum 1 charged with uniformelectricity by the charged roller 2. Accordingly, an electrostaticlatent image corresponding to the Y-color image is formed on thephotoconductive drum 1. A developing bias is applied to the developingroller 5 of the Y-developer (4Y) so that the Y-color toner image isformed on the photoconductive drum 1. Then, the Y-color toner image istransferred to the intermediate transfer belt 6 by the first transferbias applied to the first transfer roller 7.

After the Y-color toner image corresponding to a single page iscompletely transferred to the intermediate transfer belt 6, the exposureunit 2 outputs an optical signal corresponding to the M-color imageinformation to the photoconductive drum 1 re-charged with the uniformelectricity by the charged roller 2, so that an electrostatic latentimage corresponding to the M-color image is formed on thephotoconductive drum 1. The M-color toner is applied to theelectrostatic latent image by the M-color developer (4M), and is thendeveloped. The M-color toner image formed on the photoconductive drum 1is transferred to the intermediate transfer belt 6, so that the M-colortoner image overlaps with the previously transferred Y-color tonerimage.

The above-mentioned process may also be applied to the C and K colors.In this case, the color toner image is formed in which the Y-, M-, C-,and K-toner images overlap each other. This color toner image istransferred to the print medium P from the intermediate transfer belt 6and the second transfer roller 8 by the second transfer bias. The fixingunit 9 applies heat and pressure on the color toner image, so that theimage is fixed onto the print medium P.

According to aspects of the present invention, the above-mentioned colorimage forming apparatus prevents a composite black text image from beingdistorted by mis-registration, prevents a blurred color from beinggenerated at a boundary of the composite black text, and preventsseveral color dots from dispersing in various directions at the boundaryof the composite black text. In the case of the document printing mode,some colors are printed on a single print medium several times in orderto print a desired image, as opposed to a black-and-white printer (i.e.,a mono printer) in which the black color is printed on a single printmedium only once. As a result of the plurality of colors that areprinted to be overlapped with each other on only one print medium, thecolor image forming apparatus has difficulty in correctly printingindividual colors at desired locations due to a variety of factors. Thisproblem is called a mis-registration. Aspects of the present inventionprovide a software method (instead of using a hardware method) forpre-processing printing data so that the print data can be printedsimilar to an original image irrespective of mechanical errors.

FIG. 2 is a flow chart illustrating a method of controlling a colorimage forming apparatus according to an example embodiment of thepresent invention. Referring to FIG. 2, the color image formingapparatus receives color print data required to print the color image,and receives CMYK data of 8 bits in operation S100. Then, the colorimage forming apparatus determines whether the received color print datacorresponds to a composite black text in operation S101. If the receivedcolor print data is determined to correspond to the composite black text(operation S101), the color image forming apparatus searches for aboundary area of the composite black text using a Laplacian filter inoperation S102, and performs image-processing of the boundary area so asto prevent the image from being distorted by mis-registration inoperation S103. If the received color print data is determined to notcorrespond to the composite black text (operation S101), the color imageforming apparatus extracts a specific area in which the directions ofedges among colors are opposite to each other in operation S104, so thata boundary area among colors is searched for. The color channels of theboundary area are extended and emphasized, and the boundary area isimage-processed so as to not be distorted by mis-registration inoperation S105.

FIG. 3 is a flow chart illustrating a method of determining a compositeblack text of FIG. 2 according to an example embodiment of the presentinvention. Referring to FIG. 3, a 3×3 window for the C, M, Y, and Kchannels is established in operation S110. A bitmap is created by athreshold value in operation S111 in order to determine whether thepatterns among the C, M, Y, and K channels are identical to each otherin operation S112. If the patterns among the C, M, Y, and K channels areidentical to each other (operation S112), it is determined whether the Kchannel is flat in operation S113. If the K channel is flat (operationS113), the composite black text is determined in operation S114. In moredetail, if the patterns among the C, M, Y, and K channels are identicalto each other due to characteristics of the composite black text, the Kchannel is flat. If the K channel is not flat (operation S113), anon-composite black text is determined in operation S115.

FIG. 4 is a flow chart illustrating a method of determining a compositeblack text according to an example embodiment of the present invention.Referring to FIG. 4, in association with the 3×3 window of the C, M, Y,and K channels, a 3×3 bitmap associated with the 3×3 window is createdby a threshold value in operation S120.

An average value of window values formed at ON locations of theK-channel bitmap from among the K-channel 3×3 window values iscalculated in operation S121. A variance value (Variance_K) iscalculated on the basis of the average value and the ON pixel valuescontained in the window in operation S122.

Thereafter, it is determined whether the C, M, Y, and K channel bitmapscreated in operation S120 have the same patterns. Specifically, if allfour channels are switched ON or all four channels are switched OFF atall pixel locations of the 3×3 window, it is determined that thepatterns are equal to each other in operation S123. Also, in operationS124, it is determined whether the K channel is flat according to thevariance value (Variance_K) calculated in operation S122. Specifically,if the variance value (Variance_K) is less than a predetermined value(Threshold_Flat) in operation S124, it is determined that the K channelis flat.

If the aforementioned two conditions are satisfied (all four channelsare “On” or “Off” and the K channel is flat), the composite black textis determined in operation S125. In more detail, the color image formingapparatus searches for the composite black text by estimating a degreeof coincidence among patterns of the four channels and a degree ofplanarization of the K channel. In particular, the color image formingapparatus can search for the composite black text because the compositeblack text has the same C, M, Y, and K patterns and has a low deviationamong dot levels of dot-ON locations of the “ON” K channel. Therefore,the color image forming apparatus searches for the composite black textusing characteristics of the composite black text.

If either one of the above-mentioned two conditions is not satisfied(all four channels are “On” or “Off” or the K channel is flat), thenon-composite black text is determined in operation S126.

FIG. 5 is a conceptual diagram illustrating a method of determining acomposite black text in a 3×3 window according to an example embodimentof the present invention. As can be seen from FIG. 5, the C, M, Y, and Kchannels overlap with each other to form the composite black area. TheC, M, Y, and K channels have the same “dot-ON” and “dot-OFF” locations,and have a low deviation among dot values of the ON area of the Kchannel. Accordingly, the composite black text can be determined basedon the C, M, Y, and K channels.

Accordingly, if the composite black text is determined, the boundaryarea of the composite black text is found using the Laplacian filter. Ascan be seen from FIG. 6, the C, M, and Y areas are removed from thefound boundary area. Only the K channel is left on the outermostboundary of the composite black text. In this case, the K channel iscompensated as much as the density of the removed C, M, and Y areas. Inother words, the C, M, and Y values are respectively set to “0” at theboundary area and the K value is set to K+density (CMY)*W, so that theimage distortion is compensated.

FIG. 7 shows a comparison result between a first image created beforethe image distortion compensation of the composite black text “quick”and a second image after the image distortion compensation of thecomposite black text “quick” according to an example embodiment of thepresent invention. As can be seen from FIG. 7, the first image includesthe C, M, and Y colors, which disperse in the vicinity of the compositeblack text by the mis-registration. However, in the case of the secondimage including the compensated image distortion, the image distortioncaused by the C, M, and Y channel colors is lower in the vicinity of thecomposite black text.

As is apparent from the above description, aspects of the presentinvention prevent an image distortion from occurring in the vicinity ofcomposite black text due to mis-registration, in which C, M, Y, and Kchannels are printed in incorrect locations. Specifically, aspects ofthe present invention remove the C, M, and Y areas from the boundaryarea of the composite black text to reduce the C, M, and Y areas.Furthermore, aspects of the present invention allow only the K channelto be left on the outermost boundary of the composite black text andcompensate for the K channel by the density of the reduced C, M, and Yareas. Accordingly, the image distortion generated at the boundary ofthe composite black text is compensated for, resulting in increasedimage quality of the printed image of the composite black text.

While there have been illustrated and described what are considered tobe example embodiments of the present invention, it will be understoodby those skilled in the art and as technology develops that variouschanges and modifications, may be made, and equivalents may besubstituted for elements thereof without departing from the true scopeof the present invention. Many modifications, permutations, additionsand sub-combinations may be made to adapt the teachings of the presentinvention to a particular situation without departing from the scopethereof. For example, determining whether the K channel is flat mayoccur before or simultaneous to determining whether all four channelsare ON or OFF. Accordingly, it is intended, therefore, that the presentinvention not be limited to the various example embodiments disclosed,but that the present invention includes all embodiments falling withinthe scope of the appended claims.

1. A method of printing a composite black text with a color imageforming apparatus using cyan (C), magenta (M), yellow (Y), and black (K)channels, the method comprising: determining whether print datacorresponds to a composite black text; searching for a boundary area inthe print data if the print data is determined to correspond to thecomposite black text; and reducing a C channel, an M channel, and a Ychannel at the boundary area, and compensating for a K channel by adensity of the reduced C, M, and Y channels if the print data isdetermined to correspond to the composite black text.
 2. The method asclaimed in claim 1, wherein the determining of whether the print datacorresponds to the composite black text comprises: establishing a 3×3window associated with C, M, Y, and K channels of the print data;generating C, M, Y, and K bitmaps based on the 3×3 window; determining,according to the generated C, M, Y, and K bitmaps, whether the Cchannel, the M channel, the Y channel, and the K channel have a samepattern and whether the K channel is flat; and determining that theprint data corresponds to the composite black text if the C channel, theM channel, the Y channel, and the K channel are determined to have thesame pattern and the K channel is determined to be flat.
 3. The methodas claimed in claim 2, wherein the generating of the C, M, Y, and Kbitmaps comprises generating 3×3 C, M, Y, and K bitmaps using athreshold value.
 4. The method as claimed in claim 2, wherein thedetermining of whether the C channel, the M channel, the Y channel, andthe K channel have the same pattern comprises: determining that the C,M, Y, and K channels have the same pattern if the C, M, Y, and Kchannels are simultaneously in either a “dot ON” status or a “dot OFF”status at all pixel locations of the 3×3 window.
 5. The method asclaimed in claim 2, wherein the determining of whether the K channel isflat comprises: calculating an average value of window values generatedat “dot ON” locations of the K channel bitmap from among valuescontained in the 3×3 window; calculating a variance value from “dot ON”pixel values of the 3×3 window and the calculated average value;determining whether the variance value is less than a predeterminedvalue; and determining that the K channel is flat when the variancevalue is less than the predetermined value.
 6. The method as claimed inclaim 1, wherein the reducing of the C channel, the M channel, and the Ychannel at the boundary area comprises reducing the C channel, the Mchannel, and the Y channel to 0 at the boundary area.
 7. The method asclaimed in claim 6, wherein the compensating for the K channel comprisescompensating for a K channel located at an outermost area of thecomposite black text by the density of the reduced C, M, and Y channels.8. The method as claimed in claim 1, wherein the compensating for the Kchannel comprises compensating for a K channel located at an outermostarea of the composite black text by the density of the reduced C, M, andY channels.
 9. The method as claimed in claim 1, wherein the boundaryarea is searched for using a Laplacian filter.
 10. The method as claimedin claim 1, wherein the determining of whether the print datacorresponds to the composite black text comprises determiningsimilarities between the C channel, the M channel, the Y channel, andthe K channel, and determining a degree of planarization of the Kchannel.
 11. A method of printing a composite black text with a colorimage forming apparatus using cyan (C), magenta (M), yellow (Y), andblack (K) channels, the method comprising: determining whether patternsof C, M, Y, and K channels in print data are identical to each other,and determining whether the K channel is flat; determining that theprint data corresponds to a composite black text if the patterns of theC, M, Y, and K channels are identical and the K channel is flat; if theprint data corresponds to the composite black text, searching for aboundary area in the print data; and reducing the C channel, the Mchannel, and the Y channel at the boundary area, and compensating forthe K channel by a density of the reduced C, M, and Y channels if theprint data corresponds to the composite black text.
 12. The method asclaimed in claim 11, further comprising: establishing a 3×3 windowassociated with the C, M, Y, and K channels; generating C, M, Y, and Kbitmaps based on the 3×3 window; wherein the determining of whether thepatterns of the C, M, Y, and K channels are identical and whether the Kchannel is flat comprises determining whether the patterns of the C, M,Y, and K channels are identical and whether the K channel is flataccording to the generated C, M, Y, and K bitmaps.
 13. The method asclaimed in claim 12, wherein the determining of whether the patterns ofthe C, M, Y, and K channels are identical according to the generated C,M, Y, and K bitmaps comprises: determining that the patterns of the C,M, Y, and K channels are identical if the C, M, Y, and K channels aresimultaneously in either a “dot ON” status or a “dot OFF” status at allpixel locations of the 3×3 window.
 14. The method as claimed in claim12, wherein the determining of whether the K channel is flat accordingto the generated C, M, Y, and K bitmaps comprises: calculating anaverage value of window values generated at “dot ON” locations of the Kchannel bitmap from among values contained in the 3×3 window;calculating a variance value from “dot ON” pixel values of the 3×3window and the calculated average value; determining whether thevariance value is less than a predetermined value; and determining thatthe K channel is flat when the variance value is less than thepredetermined value.
 15. The method as claimed in claim 11, wherein thecompensating for the K channel comprises: compensating for a K channellocated at an outermost boundary area of the composite black text by thedensity of the reduced C, M, and Y channels.
 16. The method as claimedin claim 15, wherein the reducing of the C channel, the M channel, andthe Y channel comprises reducing the C channel, the M channel, and the Ychannel to 0 at the boundary area.
 17. The method as claimed in claim11, wherein the reducing of the C channel, the M channel, and the Ychannel comprises reducing the C channel, the M channel, and the Ychannel to 0 at the boundary area.
 18. A method of printing a compositeblack text with a color image forming apparatus using cyan (C), magenta(M), yellow (Y), and black (K) channels, the method comprising:searching for a boundary area in print data if the print data isdetermined to correspond to a composite black text; and reducing the Cchannel, the M channel, and the Y channel at the boundary area, andcompensating for the K channel by a density of the reduced C, M, and Ychannels if the print data is determined to correspond to the compositeblack text.
 19. A color image forming apparatus that prints a compositeblack text using cyan (C), magenta (M), yellow (Y), and black (K)channels, the apparatus comprising: a control unit to search for aboundary area in print data if the print data is determined tocorrespond to a composite black text, to reduce the C channel, the Mchannel, and the Y channel at the boundary area, and to compensate forthe K channel by a density of the reduced C, M, and Y channels.
 20. Theapparatus as claimed in claim 19, wherein the control unit determineswhether the print data corresponds to the composite black text bydetermining whether patterns of C, M, Y, and K channels in the printdata are identical to each other, and determining whether the K channelis flat, such that the print data is determined to correspond to thecomposite black text if the patterns of the C, M, Y, and K channels areidentical and the K channel is flat.
 21. The apparatus as claimed inclaim 20, wherein the control unit establishes a 3×3 window associatedwith the C, M, Y, and K channels, generates C, M, Y, and K bitmaps basedon the 3×3 window, and determines whether the patterns of the C, M, Y,and K channels are identical and whether the K channel is flat accordingto the generated C, M, Y, and K bitmaps.
 22. The apparatus as claimed inclaim 21, wherein the control unit determines that the patterns of theC, M, Y, and K channels are identical if the C, M, Y, and K channels aresimultaneously in either a “dot ON” status or a “dot OFF” status at allpixel locations of the 3×3 window.
 23. The apparatus as claimed in claim21, wherein the control unit determines whether the K channel is flat bycalculating an average value of window values generated at “dot ON”locations of the K channel bitmap from among values contained in the 3×3window, calculating a variance value from “dot ON” pixel values of the3×3 window and the calculated average value, determining whether thevariance value is less than a predetermined value, and determining thatthe K channel is flat when the variance value is less than thepredetermined value.
 24. The apparatus as claimed in claim 19, whereinthe control unit compensates for only a K channel located at anoutermost boundary area of the composite black text by a density of thereduced C, M, and Y channels.
 25. The apparatus as claimed in claim 19,wherein the control unit reduces the C channel, the M channel, and the Ychannel to 0 at the boundary area.